Small particles that scatter light and reflect solar radiation back into space. Can range in size to 10 nanometers to 10 micrometers. Can be produced naturally of be man made. More aerosols means less solar flux can enter earth (negative radiative forcing) Direct effect: reflects solar radiation away from earth. Indirect effect: increased aerosol changes the structure and behavior of clouds. 1st effect: a cloud with more aerosol will reflect more light (small particles reflect better than large ones) 2nd effect: smaller droplets take longer to get big enough to precipitate. Some aerosols are absorptive (black carbon, dust) and are positive. Ship tracks: ships release aerosols into the air and the clods above have enhanced reflectivity, appearing as white lines. Direct aerosol effect: increasing levels of aerosols provide a negative radiative forcing effect since the particles reflect/scatter incoming radiation from the earth and earths temp decreases.